The present invention relates generally to visual display applications, such as electronic ink, and, more particularly, to a molecular system that provides optical switching. Optical devices with micrometer or sub-micrometer critical dimensions may be constructed in accordance with the teachings herein.
Flexible displays made with a technology known as electronic ink, or E ink, are in the process of commercial development. While the early versions are expected to resemble simple displays that might sit by the side of a highway to warn of trouble ahead or might advertise specials at a convenience store, later versions are expected to lead to electronic books with paper-like pages and illustrations that move, newspapers that update themselves, reusable paper displays for cellular phones, disposable TV screens, and even electronic wallpaper.
There are two presently-known competing technologies: E Ink""s electrophoretic displays and Xerox""s gyricon spheres.
The electrophoretic displays are disclosed, for example, in U.S. Pat. No. 6,017,584, issued Jan. 25, 2000, and entitled xe2x80x9cMulti-Color Electrophorectic Displays and Materials for Making the Samexe2x80x9d, and in U.S. Pat. No. 6,067,185, issued May 23, 2000, and entitled xe2x80x9cProcess for Creating an Encapsulated Electrophoretic Displayxe2x80x9d.
Generally, an encapsulated electrophoretic display includes one or more species of particles that either absorb or scatter light. One example is a system in which the capsules contain one or more species of electrophoretically mobile particles dispersed in a dyed suspending medium. Another example is a system in which the capsules contain two separate species of particles suspended in a clear suspending fluid, in which one of the species of particles absorbs light (black), while the other species of particles scatters light (white). Other extensions are possible, including more than two species of particles, with or without a dye, etc. The particles are commonly solid pigments, dyed particles, or pigment/polymer composites.
The gyricon spheres are disclosed in a number of patents issued and assigned on their face to Xerox Corporation; an example of one such patent is U.S. Pat. No. 5,892,346, issued Nov. 9, 1999, and entitled xe2x80x9cFabrication of a Twisting Ball Display Having Two or More Different Kinds of Ballsxe2x80x9d.
The gyricon, also called the twisting-ball display, rotary ball display, particle display, dipolar particle light valve, etc., offers a technology for making a form of electric paper. Briefly, a gyricon is an addressable display made up of a multiplicity of optically anisotropic balls, each of which can be selectively rotated to present a desired face to an observer. Thus, in one version at least, the gyricon is a solid microsphere, hemispherically-colored black and white and having hemispherically-opposing zeta potentials. Each gyricon rotates within a dielectric oil-filled microcavity formed in the media upon exposure to an externally-applied electric field.
The primary disadvantage of both electrophoretic ink and the gyricon is poor contrast. Hemispherically-colored microspheres, or microcapsules, being fully three dimensional, have backside reflection and scattering that reduce the contrast of dark and white images reflected toward the observer.
The second disadvantage of both the electrophoretic ink and the gyricon solutions is limited image resolution. Both solutions are limited to practical microcapsule or microsphere diameters on the order of 20 to 40 micrometers. Electrophoretic ink microcapsules are limited by the need to microencapsulate sufficient pigmented colorant to provide reasonable color contrast and opacity within each microcapsule. Gyricon spheres are limited by thermal mass requirements to form microspheres from coalesced colored droplets in water. Microsphere diameters on the order of 5 to 10 micrometers are desired and common to toner colorant used in laser printers.
Each prior solution must use a low dielectric liquid (oil), rather than water. Since water is an excellent solvent for ionic species, water solutions are conductive and would collapse the electric field that otherwise allows electrophoretic movement of the colorants. The colorant switching time and voltage is dependent on oil viscosity, which is negatively impacted by lowered ambient temperature.
Finally, the prior art colorants have poor mechanical durability by virtue of their microcapsule composition. Microcapsule fabrication processes generally produce micron thin capsule walls, typically 10% of the capsule diameter, which are easily broken. This factor is why microcapsules are typically used to deliver encapulsated fluids upon application of external pressure or solvation (e.g., carbonless paper). The fragile nature of microcapsules makes them poorly suited for electronic paper applications where folding and surface contact is common.
Thus, what is needed is a molecular system that exhibits image contrast and mechanical durability commensurate with ink on paper, avoids chemical oxidation and/or reduction, permits reasonably rapid switching from a first state to a second state, is reversible to permit real-time or video rate display applications, and can be used in a variety of optical display applications, such as e-ink.
In accordance with the present invention, a switchable medium for a visual display comprising an electric field activated bi-stable or e-field dependent molecular system configured within an electric field generated by a pair of electrodes is provided. The molecular system has an electric field induced band gap change that occurs via a change (reversible or irreversible) of the extent of the conjugation in a molecule via chemical bonding change, which results in the change of the band gap. That is to say, the molecular system can undergo a bond breaking or making in the presence of an applied electric field, thereby inducing a band gap change in the molecular system, wherein in a first state, there is substantial conjugation throughout the molecular system, resulting in a relatively smaller band gap, and wherein in a second state, the substantial conjugation is destroyed, resulting in a relatively larger band gap. The changing of substantial conjugation may be accomplished in one of the following ways:
(1) charge separation or recombination accompanied by increasing or decreasing the localization of molecular electronic states; or
(2) change of the extent of conjugation of the molecular electronic states via charge separation or recombination andxcfx80-bond breaking or making.
The present invention provides field switchable molecules that can be assembled easily to make electronic ink, visual displays, electronic books, rewriteable media, and the like, in which the molecules reversibly change color when changing state (e.g., one color to a second color or transparent to color). Such applications are discussed elsewhere, and are not germane to the present invention, except to the extent that the field switchable molecules of the present invention are employed in the construction of apparatus of such applications.
Thus, the molecule is never oxidized nor reduced in the toggling of the switch. Further, the molecule exhibits image contrast and mechanical durability commensurate with ink on paper. Also, the part of the molecule that moves is quite small and free of viscous drag forces, so the switching time should be very fast.
A primary advantage of the molecular system of the present invention is improved contrast. Because the colorant of the present invention is molecular and thus effectively monoplanar, there should be no backside reflection or scattering from the colorant. The background color (e.g., white) of the electronic media is provided by the media substrate or substrate coating and is not compromised by the switchable colorant.
A second advantage of the present invention is improved resolution. In this instance, resolution is only limited by the addressing scheme, since the colorant has molecular dimensions.
A third advantage is that the switching may have a longer lifetime since the switching only involves the charge separation and recombination inside molecules.
Finally, each molecule of the present invention will latch to stabilize one or the other of its color states.